Facebook needs to determine a way to score and moderate without annihilating individual writers selectively.
Even just cause it frequently has mistakes. We don't need to recreate jail statistics with guilt and innocence so pronounced by secret individual actions. We can record it.
AFAIK, it is more complex than that, and the description of the author is correct:
A "bad" comment is moderated away for everyone but its author and his/her the friends.
An administrator of a Facebook page can hide a comment made on that page to all but the commenter himself and his friends. The other comments that the author saw disappear were most likely hidden while he was logging in/out. No conspiracy, just a spineless Facebook Pages feature.